1. Technical Field
The present disclosure relates to a Coriolis mass flowmeter.
2. Related Art
Recently, a Coriolis mass flowmeter for measuring the mass flow rate, the density, the volume flow rate, etc., of a fluid has been often used. According to the Coriolis mass flowmeter, a tube through which a fluid flows is vibrated and the mass flow rate of the fluid flowing through the tube is measured from the phase difference between vibration detection signals at different two upstream and downstream points of the tube. Paying attention to a detection method of a phase difference of the Coriolis mass flowmeter, the Coriolis mass flowmeter is roughly classified into that using an analog detection method and a digital detection method.
The analog detection method is a method of finding the point in time at which each of the amplitudes of detection signals obtained from an upstream sensor (pickup coil) and a downstream sensor become 0 (zero cross point) and detecting the phase difference between the detection signals from the time difference between the zero cross points.
In contrast, the digital detection method is a detection method of sampling detection signals obtained in an upstream sensor and a downstream sensor at the same timing, converting the signals into digital signals, and performing predetermined signal processing for the digital signals, thereby detecting the phase difference between the detection signals. As the signal processing performed for detecting the phase difference between the detection signals, DFT (Discrete Fourier Transform) processing and Hilbert transformation processing are known.
For example, JP-A-2009-063382 (especially, see page 4, FIG. 1) discloses a Coriolis mass flowmeter adopting a digital detection method of this kind. FIG. 12 is a block diagram showing a general configuration example of a Coriolis mass flowmeter 500 according to a conventional example. The Coriolis mass flowmeter 500 shown in FIG. 12 includes a detector 60 and a converter 75.
The detector 60 vibrates a tube 15 (pipe line) through which a fluid to be measured flows and detects upstream and downstream vibration frequencies, vibration phases, and fluid temperatures. The detector 60 includes support members 16 and 17, an exciter 61, an upstream sensor 62, a downstream sensor 63, and a temperature sensor 64. The support members 16 and 17 fix and support the tube 15. The fluid in the tube 15 flows from the left to the right of the plane of the drawing.
In the example, the exciter 61 is placed above a midpoint of the support members 16 and 17, excites the tube based on a vibration control signal S, and vibrates the tube 15. The upstream sensor 62 is placed on the right side of the support member 16 and in the proximity of the tube 15, detects vibration of the upstream fluid, and outputs an upstream coil signal S1 (pickup signal) to the converter 75.
The downstream sensor 62 is placed on the left side of the support member 17 and in the proximity of the tube 15, detects vibration of the downstream fluid, and outputs a downstream coil signal S2 (pickup signal) to the converter 75. In the example, the temperature sensor 64 is provided on the right side of the support member 17 and in contact with the tube 15, detects the temperature of the fluid (detector 60), and outputs a temperature detection signal S9 to the converter 75. The converter is connected to the exciter 61, the upstream sensor 62, the downstream sensor 63, and the temperature sensor 64.
FIG. 13 is a schematic representation showing a vibration mode example of the tube 15. According to the vibration mode example of the tube 15 shown in FIG. 13, when a vibration control signal S7 is supplied from the converter 75 to the exciter 61, the tube 15 is vibrated in a primary mode indicated by signs M1 and M2 in FIG. 13, for example. The primary mode refers to a vibration state appearing only in portions where nodes of vibration are fixed and supported by the support members 16 and 17. When a fluid flows into the tube 15 in such a vibration state, the tube 15 vibrates in a secondary mode indicated by signs M3 and M4 in the figure, for example. The secondary mode refers to a vibration state appearing in portions where nodes of vibration are fixed and supported by the support members 16 and 17 and an intermediate position. In fact, the tube 15 vibrates a composite vibration mode provided by superposing the two types of vibration modes.
FIG. 14 is a block diagram showing an internal configuration example of the Coriolis mass flowmeter 500. The detector 60 shown in FIG. 14 includes an upstream coil L1, a downstream coil L2, a drive coil L3, and a resistance temperature detector RTD. The drive coil L3 excites the tube 15 based on the vibration control signal S7 and induces vibration. The upstream coil L1 detects vibration of the tube 15 and outputs the upstream coil signal S1. The downstream coil L2 detects vibration of the tube 15 and outputs the downstream coil signal S2. The resistance temperature detector RTD measures the temperature of the tube and outputs the temperature detection signal S9.
The converter 75 finds the phase difference between the upstream coil signal S1 and the downstream coil signal S2 detected by the detector 60 and finds the mass flow rate of the fluid flowing through the tube 15. The converter 75 includes input amplifiers 1 and 2, a switch circuit 3, A/D converters 4 and 9, a digital signal processing circuit 5, a CPU 6, a drive circuit 7, an RTD drive circuit 8, output circuits 10 and 11, a display 12, a frequency measuring circuit 13, and a timing generator 85.
The input amplifier 1 is connected to the upstream coil L1, amplifies the upstream coil signal S1, and outputs the post-amplified upstream coil signal S1 to the A/D converter 4. The switch circuit 3 for switching a path is connected to the downstream coil L2 and switches the path so as to select an upstream coil signal S11 after branch or the downstream coil signal S2 from the detector 60. The switch circuit 3 is used to execute a function of zero point signal compensation (hereinafter, simply called zero compensation) of the portion of the input amplifier 1, 2.
The input amplifier 2 is connected to the switch circuit 3, amplifies the upstream coil signal S11 or the downstream coil signal S2, and outputs the post-amplified upstream coil signal S11 or downstream coil signal S2 to the A/D converter 4. The A/D converter 4 converts the post-amplified upstream coil signal S1, S1 into a digital signal S1′, S1′, and outputs the digital upstream coil signal S1′, S1′ to the digital signal processing circuit 5.
The A/D converter 4 converts the post-amplified upstream coil signal S11 or downstream coil signal S2 into digital signals S11′ and S2′ and outputs the digital upstream coil signal S11′ or the digital downstream coil signal S2′ to the digital signal processing circuit 5. The A/D converter 4 uses a two-channel A/D converter having a function capable of sampling the upstream coil signal S1 and the upstream coil signal S11 or the downstream coil signal S2 at the same time.
The digital signal processing circuit 5 inputs the upstream coil signal S1′ and the downstream coil signal S2′ from the A/D converter 4, calculates the phase difference between the upstream coil signal S1′ and the downstream coil signal S2′, and outputs a flow rate signal indicating the mass flow rate of the fluid. The CPU 6 converts the phase difference between the flow rate signals into a mass flow rate and performs computation of compensating for the effect caused by the temperature of the detector 60 for a flow rate signal, a density signal, etc. At the time, the CPU 6 computes process variables of mass flow rate, accumulated mass flow rate, fluid density, etc.
The drive circuit 7 positively feeds back the upstream coil signal S1 or the downstream coil signal S2 and excites the drive coil L3 at a unique oscillation frequency f0 of the tube 15 through which a fluid flows. The RTD drive circuit 8 outputs a resistance drive signal S8 for setting an operation current in the resistance temperature detector RTD. The A/D converter 9 converts the analog temperature detection signal S9 output from the resistance temperature detector RTD into a digital signal and outputs digital temperature detection data D9 to the CPU 6.
The frequency measuring circuit 13 measures the unique oscillation frequency f0 of the tube 15 excited by the drive circuit 7 and outputs frequency data D13 to the CPU 6. The oscillation frequency f0 is used for density measurement, etc., of a fluid. The unique oscillation frequency f0 of the tube 15 is calculated by the digital signal processing circuit 5 or the CPU 6. The timing generator 85 generates a switch clock signal CK and outputs the signal to the switch circuit 3. The output circuits 10 and 11 output digital flow rate data D5 indicating the flow rate of a fluid based on the mass flow rate computed by the CPU 6. The display 12 displays the flow rate, the mass, the temperature, etc., of a fluid.
FIG. 15 is a waveform chart showing a phase detection example between the upstream coil signal S1 and the downstream coil signal S2. In FIG. 15, the horizontal axis indicates time t and the vertical axis indicates signal level. In the figure, the detection phase waveform indicated by the solid line is the upstream coil signal S1 detected in the upstream coil L1 in the Coriolis mass flowmeter 500. The detection phase waveform indicated by the dotted line is the downstream coil signal S2 detected in the downstream coil L2. Up arrows denote signal sampling timings T0, T1, T2, . . . in the A/D converter 4.
According to the phase relationship between the upstream coil signal S1 and the downstream coil signal S2 shown in FIG. 15, the downstream coil signal S2 delays relative to the upstream coil signal S1 and a phase difference occurs between the upstream coil signal S1 and the downstream coil signal S2. The upstream coil signal S1 is amplified in the input amplifier 1 and the downstream coil signal S2 is amplified in the input amplifier 2 and then the signals are sampled in sampling period T and become discrete digital flow rate signals at the point in time indicated by “•” in FIG. 15.
The flow rate signals (digital data) are input and are subjected to digital signal processing in the digital signal processing circuit 5 and the phase difference between the upstream coil signal S1 and the downstream coil signal S2 is found. Since the phase difference between the upstream coil signal S1 and the downstream coil signal S2 is converted into the mass flow rate, it is required that the upstream coil signal S1 and the downstream coil signal S2 should be sampled precisely at the same time.
The upstream coil signal S1 detected in the upstream coil L1 and the downstream coil signal S2 detected in the downstream coil L2 pass through two signal processing paths where they are sampled at the same time in the input amplifiers 1 and 2 and the A/D converter 4 shown in FIG. 14. Thus, if a difference of the effect caused by ambient temperature fluctuation, etc., exists between the two signal processing paths, phase difference output as the final flow data D5 may fluctuate depending on an environmental condition. The fluctuation amount may change due to the unique frequencies of the upstream coil signal S1, the downstream coil signal S2, etc.
Further, the fluctuation amount under the same environmental condition is not necessarily constant because of secular change, etc., of the components and the wiring forming the two signal processing paths. Generally, the accuracy required for the flow rate signal (phase difference signal) of the Coriolis mass flowmeter 500 is equivalent to grad or less. Thus, it becomes necessary to compensate for the effect of the fluctuation caused by the environmental condition, the secular change, etc.
According to the Coriolis mass flowmeter 500 according to the conventional example, the switch circuit 3 selects the downstream coil signal S2 at the flow rate at the flow rate measuring time and the digital signal processing circuit 5 measures the phase difference between the upstream coil signal S1′ and the downstream coil signal S2′. At the zero compensation processing time, the switch circuit 3 selects the upstream coil signal S11 at regular time intervals at given period timing based on the clock signal CK.
Specifically, to interrupt flow rate measurement and execute zero point compensation processing, the switch circuit 3 disconnects input of the downstream coil signal S2 and switches the signal processing path of the input amplifier 2 to input of the upstream coil signal S11 and connects to the digital signal processing circuit 5 (sampling hard). The digital signal processing circuit 5 measures the phase difference between the upstream coil signal S1 flowing through the input amplifier 1 and the A/D converter 4 and the upstream coil signal S11 flowing through the input amplifier 2 and the A/D converter 4.
According to the measuring timing described above, the upstream coil signal S1 flowing through the input amplifier 1 and the upstream coil signal S11 flowing through the input amplifier 2 are input to the digital signal processing circuit 5 as the same input waveform. The digital signal processing circuit 5 executes digital signal processing such as filtering of limiting of a passage band, 90-degree phase conversion, etc. Accordingly, the phase difference between the signal processing path of the upstream coil signal S1 flowing through the input amplifier 1 and the signal processing path of the upstream coil signal S11 flowing through the input amplifier 2 can be measured. The output phase difference at this time is used for correction computation by the CPU 6 as a zero compensation value, whereby the effect of the environmental conditions, etc., of the input amplifier 2, the A/D converter 4, etc., is removed.
By the way, the Coriolis mass flowmeter 500 according to the conventional example involves the following problem:
FIGS. 16A and 16B are time charts showing an operation example of the converter 75. FIG. 16A is a time chart showing a flow rate measuring example based on the signal processing path of the upstream coil signal S1 and the signal processing path of the downstream coil signal S2 and FIG. 16B is a time chart showing a zero compensation value measurement example in the signal processing paths of the upstream coil signals S1 and S11. The horizontal axes of FIGS. 16A and 16B indicate time t. Tx shown in FIG. 16A indicates the time period over which the flow rate cannot be measured (flow rate measurement interrupt time period). Tm indicates the switch wait time of internal data of the A/D converter, filtering data, etc.).
(i) In the Coriolis mass flowmeter 500 described in JP-A-2009-063382, the phase difference between the upstream coil signal S1 and the downstream coil signal S2 for calculating a flow rate signal is the order of several mrad (milliradians). For example, to obtain a flow rate signal of 0.1% accuracy, phase difference detection of the wad (microradians) is indispensable.
According to the Coriolis mass flowmeter 500 shown in FIG. 12, it is indispensable to sample and hold the phase difference between the upstream coil signal S1 and the downstream coil signal S2 at the same timing as described with reference to FIGS. 12 and 13, and the AD converter 4 that can sample two channels at the same time becomes necessary. However, the number of types of commercially available A/D converters with completely synchronized two channel sample and hold is small and many of them are expensive.
(ii) To remove phase fluctuation in the input amplifier 1 of the upstream coil signal S1, the input amplifier 2 of the downstream coil signal S1, etc., caused by an environmental condition change, the upstream coil signal S1 is input to the input amplifier 1 and the upstream coil signal S11 is input to the input amplifier 11 by the switch circuit 3 shown in FIG. 14 and a zero compensation value must be found at regular time intervals. Therefore, there is a problem in that in the computing time of the zero compensation value, flow rate measurement cannot be executed during the Tx period shown in FIG. 16A.
In this connection, to shorten the flow rate measurement interrupt time period, a method of increasing the measurement frequency in zero compensation processing is possible, but the number of samples of zero compensation value (average value) is limited and it becomes difficult to obtain sufficient stability.
(iii) According to the Coriolis mass flowmeter 500, when the upstream coil signal S11 and the downstream coil signal S2 are switched by the switch circuit 3, zero compensation value or flow rate measurement cannot immediately be executed. Generally, a sequence of filtering to limit a passage band, etc., is performed in measurement of the phase difference corresponding to the flow rate or the zero compensation value. In the processing, a flow rate signal before switch is held for filtering just after switch of the upstream coil signal S1.
Therefore, flow rate measurement must be awaited by the time period Tm shown in FIG. 16 (A) until the flow rate signal used for the filtering is replaced with a flow rate signal obtained after switch. In the A/D converter 4, just after switch, a digital flow rate signal before switch is held and a measure of wait time occurs until the effect of the flow rate signal before switch disappears.
iv. When measurement of the upstream coil signals S1 and S11 involved in the two amplifiers 1 and 2 is executed once every several seconds to several ten seconds, each zero compensation value calculated in the digital signal processing circuit 5 has fluctuation. When the measurement error of the zero compensation value becomes large, consequently an error of output after zero compensation becomes large.
To calculate a stable zero compensation value, fluctuation needs to be made sufficiently small and a method of increasing the switch frequency of the upstream coil signal S11 and the downstream coil signal S2 or a method of prolonging the measurement time period of a zero compensation value and increasing the number of pieces of data to be averaged is possible. However, there is a problem in that the flow rate measurement interrupt time period Tx described above is prolonged.